Effects of short circuit faults

A short circuit fault is an abnormal low-impedance connection between phases or between phase and earth, which makes very large currents flow.

Effects of short circuit faults

• Heavy current: fault current may be 5 to 20 times full-load current; $I^2R$ heating can damage windings, cables and insulation.
• Mechanical forces: electromagnetic force is proportional to $I^2$, so bus bars, transformer and generator windings can be bent or broken.
• Voltage dip: voltage near the fault falls sharply, so motors may stall and sensitive loads trip.
• Loss of stability: electrical output of generators falls while mechanical input stays the same, so machines may lose synchronism.
• Fire and arcing: arcs at the fault can cause fire and explosion, and put people in danger.
• Unbalance: unsymmetrical faults produce negative sequence currents that overheat the rotors of machines.
• Interruption of supply to consumers when the faulty section is disconnected.

Importance of fault calculations

1. Circuit breaker selection: the breaking capacity (MVA, kA) and making capacity of breakers and fuses are chosen from the maximum fault level.
2. Relay setting and coordination: the minimum and maximum fault currents are needed to set protective relays so they operate selectively.
3. Equipment rating: bus bars, CTs, cables and switchgear must withstand the thermal and mechanical stress of the fault current.
4. Design of reactors and grounding: to size current-limiting reactors and neutral grounding impedance that limit the fault level.
5. Stability studies: fault conditions are used to check transient stability and critical clearing time.
6. Planning: to check how new generators or interconnections change the fault level of existing substations.